94 PALEONTOLOGY 



an even larger number have been collected from the Miocene of 

 Florissant. 



The earliest known Aculeate Hymenoptera date from the 

 Eocene j^eriod, and it is evident that they were one of the latest 

 of the great groups to be evolved. Both Parasitica and Aculeata 

 are abundant as fossils ; Chalcids of the genus Eurytoma are 

 described from the Eocene of Green River, and all the great 

 groups of the parasitic forms were existing in the Lower Oligocene. 

 Considerably over 300 species of ants are known from Tertiary 

 strata, and among them male, female and worker castes were 

 differentiated much as to-day. Wheeler has remarked upon the 

 absence of polymorphism among the worker caste, and it appears 

 that differences which separate individuals into true workers and 

 soldiers were not evident until the Pleistocene. 



The higher Diptera occur as fossils in Eocene rocks and became 

 abundant in the Oligocene. Syrphidse are especially prevalent as 

 fossils, and this family, together with the Tachinida^, were already 

 existing in the Eocene period of the Green River in Wyoming. 



Little needs to be said of the remaining orders, since most are 

 represented in Tertiary times, including the smaller groups 

 Embiojitera, Strepsiptera and Aphaniptera. 



The Phylogeny of Recent and Fossil Orders of Insects 



The earliest fossil remains of insects found in Carboniferous 

 rocks were exopterygote forms endowed with large and tolerably 

 efficient wings. Along with them are fragments of contem- 

 poraneous nymphs, thus indicating that these insects underwent 

 incomplete metamorphosis. Many of these ancient insects are 

 grouped to form the order PaLneodictyoptera which comprises a 

 large assemblage of generalised forms divisible into a number of 

 families. New genera are frequently being discovered and 

 knowledge of the order has greatly developed in recent years. A 

 revisional monograph is much needed to-day wherein the vena- 

 tional features of the earlier described forms are re-studied in the 

 light of more recent knowledge. Much the same condition of 

 affairs prevails with regard to other Palaeozoic insects and the 

 limits of some of the orders require re-definition. There is 



